The invention relates to the continuous casting of metals, more specifically the casting of strips of small thickness directly from liquid metal.
The continuous casting of metal strips, in particular of strips made from carbon steel or stainless steel, with a thickness of a few mm, and preferably thicknesses of less than 10 mm, directly from liquid metal is well-known in the literature.
The casting of a metal strip is generally carried out on an installation for what is known as twin-roll casting. Installations of this type comprise as their main components, which form the permanent mold and in which the strip solidifies, two rolls with substantially horizontal axes which are arranged opposite one another and are driven in rotation in opposite directions about their respective axes. These rolls are strongly cooled on the inside, and their lateral surfaces define a casting mold with moveable walls, at which solidification of the strip in the form of two strand shells commences; these strand shells develop further starting from the roll surfaces. These stand shells converge so as to meet substantially at the “kissing point”, i.e. the zone in which the surfaces of the rolls are closest together, with a distance between them which corresponds to the thickness of the desired strip, in order to form the strip which is drawn continuously out of the casting mold. The casting space is laterally delimited by two refractory walls which are placed against the flat ends of the rolls or are held at a very short distance therefrom, in order to avoid leakage of metal. This process is described in more detail, for example, in document EP-A 0 698 433.
It is also conceivable to produce thin metal strips by the liquid metal being applied to the rotating lateral surface of a single cooled casting roll on which the metal solidifies. This makes it possible to obtain a strip with a thickness which is generally less than that produced by casting between two rolls.
The strips cast using these processes are generally intended for cold-rolling in order to obtain metal sheets with standard thicknesses. They may previously also be subjected to hot-rolling, which is intended to alter microstructures and close up porosities which may form in the core of the strip as it solidifies. From an economic point of view, it is-particularly advantageous for this hot-rolling to be carried out using a rolling stand which is positioned along the conveying path for the strip which has just been cast.
The strips cast in this way generally have to be subjected to strip-edge trimming to a width of approximately 15 mm before they are cold-rolled or also preferably before they are first coiled after casting. This is because these strip edges are formed from strand shell sections which have solidified under different conditions than those which have led to solidification of the more central parts of the strip, in particular on account of the proximity of the lateral closure walls, which have a tendency to cool the metal more quickly than normal. Possible differences between the cooling actions effected by the rolls between their edges and their more central parts may also occur. The cooling may be greater at the edges than in the center and vice versa, depending on the design of the rolls. These differences, which are often difficult to control during the solidification and cooling process between the edges and the remainder of the strip, often cause differences in the mechanical properties of the strip along its width. The metal sheets produced under these conditions may therefore have inhomogeneous properties, which needs to be avoided.
The strip edges may also be irregular in shape, which leads to inhomogeneities in the mechanical properties of the metal sheets during the subsequent rolling operations, in particular on account of the local differences in the degree of reduction obtained. On the other hand, if leaks of liquid metal occur between the casting rolls and the lateral closure walls, the strip edges become uneven, and this unevenness needs to be eliminated before the cold-rolling in order not to adversely affect the surface of the roll cylinders.
For all these reasons, the strip edges are trimmed at a given time during the processing of the strips, with the aid of trimming shears or any other tool which is conventionally used in installations for processing strips which have been produced by standard processes.
In principle, it is advantageous for the strip edges to be trimmed as early as possible, for example before the strip is coiled following its casting. If at this stage the edges are significantly warmer than the remainder of the strip, however, there is a risk of them having mechanical properties which make it more difficult for them to be completely separated from the strip by the shears, in particular if the width of the hot zone does not precisely correspond to the width of the edges which are to be cut. This problem may be exacerbated if hot-rolling of the strip prior to trimming of its edges is performed in one line. This is because this hot-rolling is often preceded by a step of reheating the strip in an induction furnace. During this reheating, the field lines tend to concentrate at the edges of the strips, which leads to temperatures at the edges which are higher than the temperatures prevailing at the remainder of the strip. This homogeneity persists after the hot-rolling and makes subsequent correct trimming of the strip edges difficult.